Replace ComboIterator with std::set<std::vector<int>> via combinations.hpp

Since the number of generators never exceeds 6, the maximum number of combinations is (6, 3) = 20, so the space optimizations of using an iterator is mute.

Doing this way also allows to use set_difference and set_union to deal with collections of subgroups. This is not easily possible otherwise.

vis/include/combinations.hpp

@@ -0,0 +1,44 @@
+#pragma once
+
+#include <vector>
+#include <set>
+#include <algorithm>
+
+template<class V>
+V select(const V &options, const std::vector<bool> &mask, size_t count) {
+    V result;
+    result.reserve(count);
+
+    for (int i = 0; i < mask.size(); ++i) {
+        if (mask[i]) result.push_back(options[i]);
+    }
+
+    return result;
+}
+
+template<class V>
+std::set<V> combinations(const V &options, size_t count) {
+    std::set<V> result;
+
+    std::vector<bool> mask(options.size(), false);
+    std::fill(mask.begin(), mask.begin() + count, true);
+
+    do {
+        result.insert(select(options, mask, count));
+    } while (std::next_permutation(mask.begin(), mask.end(), std::greater<>()));
+
+    return result;
+}
+
+template<class V>
+std::set<V> difference(const std::set<V> &a, const std::set<V> &b) {
+    std::set<V> result;
+
+    std::set_difference(
+        a.begin(), a.end(),
+        b.begin(), b.end(),
+        std::inserter(result, result.end())
+    );
+
+    return result;
+}